1. Field of the Invention
The present invention relates to novel compounds useful in liquid crystal displays, their preparation and novel intermediates.
2. Description of the Prior Art
Liquid crystal mixtures having a positive dielectric constant anisotropy find particular utility in electro-optic cells. Examples of such liquid crystal (L.C.) mixtures are described in DT-PS- No. 2,321,632. An important parameter for the operation of LC-displays is the width of the crystalline mesophase as determined by the difference between the transformation temperature crystalline/liquid crystalline (i.e., C/L temperature) or the crystalline/nematic (C/N) temperature, on one hand and the liquid crystalline/isotropic (LC/I) transformation temperature or nematic/isotropic (N/I) transformation temperature on the other hand. Also important is the threshold voltage, V.sub.s, which determines the voltage necessary for the operation of the L.C. cell which is dependent on the magnitude of the positive dielectric constant anisotropy. The consistency or viscosity as well as the chemical and physical stability of liquid crystal mixture also influences the operation of L.C. displays.
These parameters may be partially influenced by the particular components and their concentration in the liquid crystal mixture. However, many of the components or additives disclosed in the prior art which may favorably affect one of the parameters, e.g., the width of the mesophase; have deleterious effect on one or more of the other parameters; e.g., the magnitude of the positive dielectric anisotropy.
The complexity of liquid crystal mixtures increases as the number of components increases. The increasing complexity can lead to irreversible changes during the use of such mixtures, for example, short-term supercooling may cause crystallization or chromotagraphic-type dissociation phenomena.
A satisfactory solution to this problem has not been found and research continues for L.C. compounds, L.C. mixtures and components which impart the desired operating characteristics with the fewest possible active compounds wherein compounds which favorably influence one parameter do not adversely effect another parameter.
As additives for raising the clearing point of LC-mixtures with overall positive DC-anistropy or compensating the clearing point depressing action of other components, i.e., for generally stabilizing the mesophase - there is known a group of aromatic compounds with three nuclei in a linear arrangement according to the formula (10) ##STR2## wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 have the significance given below and the rings A, B, C carry additional substituents as necessary:
According to DT-OS No. 2,321,623 these are used as additives for LC-mixtures compounds of the formula (10) in which the R.sup.1 and R.sup.4 groups indicate linear-chain alkyl- or alkoxy-radicals with 1-8 C-atoms, and the groups R.sup.2 and R.sup.3 are carbonyloxy-groups ##STR3##
DT-OS No. 2,450,088 discloses additives for LC-mixtures compounds of the formula (10) in which R.sup.1 and R.sup.4 are linear-chain alkyl- or alkoxy-groups with 1-8 C-atoms, R.sup.2 is a single bond and R.sup.3 is the carbonyloxy-radical or the oxycarbonyl one ##STR4##
DT-OS No. 2,502,904 also discloses additives for LC-mixtures, among others, such compounds of the formula (10) in which R.sup.1 is an alkyl-, alkoxy-, acyloxy- or alkoxycarbonyloxy-radical, R.sup.4 is the same as R.sup.1 or is the nitrile-group and R.sup.2 and R.sup.3 are carbonyloxy-groups or single bonds.
In DT-OS No. 2,545,121 there are disclosed as additives for LC-mixtures such compounds of the formula (10) in which
a. R.sup.1 is an alkyl-radical with 4-8 C-atoms, R.sup.2 is a single bond, R.sup.3 is the carbonyloxy-group and R.sup.4 is the nitrile-group, or PA1 b. R.sup.1 is an alkoxy-radical, R.sup.2 is the carbonyloxy-group, R.sup.3 is the oxycarbonyl-group and R.sup.4 is an alkoxy-radical, or PA1 c. R.sup.1 is an alkoxy-radical, R.sup.2 is the methinazo-group (--CH.dbd.N--), R.sup.3 is the azomethin-group (--N.dbd.HC--) and R.sup.4 is an alkoxy-radical.
In connection with the compounds (b.) and (c.) it is pointed out in DT-OS No. 2,545,121 that these, in contrast to the compounds (a.), have a negative dielectric constant anisotropy and are not suited to field effect cells.
U.S. Pat. No. 3,951,846 also discloses compounds of the formula (10) described as being suited for nematic LC-displays, where R.sup.1 is the hydrogen atom or an alkyl-, alkoxy-, acyloxy- or alkyloxy-carbonyloxy-radical, R.sup.2 is the carbonyloxy-group, R.sup.3 is a single bond and R.sup.4 is the nitrile-group.
These known compounds do permit more or less stabilization or extension of the mesophase to higher temperatures, but do not possess a positive dielectric anisotropy or too small a positive dielectric constant anisotropy. For the commercially significant known LC-displays with twisted nematic phases (field-effect rotation cell), LC-mixtures with overall high positive dielectric constant anisotropy, .DELTA. .epsilon., in the range of 10-30 or more would be advantageous.
A need therefore continues to exist for liquid crystal compounds and mixtures having a positive dielectric constant anisotropy, high clearing points and wide mesophases.